The present invention relates to separating high boiling constituents from a gaseous feed mixture containing high and low boiling constituents. More specifically, the present invention relates to the separation of high molecular weight hydrocarbons from a natural gas feed containing high and low molecular weight hydrocarbons.
The problems associated with prior art systems for separating higher and lower boiling constituents of mixture thereof is best illustrated by the separation of high and low boiling hydrocarbons of natural gas. Natural gas, as it is received from a subsurface formation, generally is not suitable for direct use without some processing, since it contains CO.sub.2, H.sub.2 S and water as contaminants. The processing operations carried out in a natural gas plant are to first remove CO.sub.2 and then pass the gas through a dehydration system to remove water. The amount of water CO.sub.2 and H.sub.2 S contained in the gas vary considerably but, in any event most gases contain significant amounts of these contaminants. In cryogenic separation techniques, the water and CO.sub.2 are removed prior to cryogenic separation and the H.sub.2 S is removed during the cryogenic separation, usually by means of fractional distillation. The preliminary dehydration and CO.sub.2 removal, of course, add to the cost of the operation. Therefore, it would be highly desirable if one of these operations, for example, the removal of CO.sub.2, could be eliminated. After removal of water, CO.sub.2 and H.sub.2 S, the resulting gas can then be used as a fuel. However, such gases generally contain varying but significant amounts of higher molecular weight components, such as ethane and, to a lesser extent, propane, butanes and higher molecular weight hydrocarbons. The ethane and higher molecular weight hydrocarbons contribute relatively little heating value to the natural gas and accordingly, these materials have a significantly greater value as chemical feedstocks than as a fuel.
The natural gas feed to a natural gas plant will generally be near atmospheric temperature and at an elevated pressure substantially above atmospheric pressure, either as it is produced from the gas formation or as a result of the compression thereof. Therefore, it has long been known to separate ethane and higher molecular weight hydrocarbons from methane by a combination of plural cooling stages and at least one expansion stage and separating the cooled and expanded fluid by fractional distillation in a "demethanizer" to produce a vapor stream substantially higher in methane contact than the original gas and a liquid stream substantially higher in ethane and higher hydrocarbons than the original gas. An effective system of this type is shown, for example, in U.S. Pat. No. 4,322,225. The system of this patent utilizes two expansion stages in series, which is generally considered more efficient than a single stage system. However, even multiple stage expansion systems are inefficient in the removal of ethane and higher hydrocarbons. In addition, since the expansion stage or stages are connected to and drive compression stages, utilized for various purposes within the natural gas processing system, the energy generated by the expanders is usually inadequate to handle all of the pressure requirements and the refrigeration needs of the overall plant. Consequently, it would also be highly desirable, from an energy saving standpoint, to increase the power output of the expanders. Also, the demethanizer either must be rather large and/or a given size demethanizer is limited in its throughput capacity. Finally, the amounts of ethane retained in the separated methane stream is often higher than desirable.